Nearly half of all cases of acute encephalitis did not have an identifiable cause. Autoimmune (antibody-mediated) etiology has gained more recognition
over the recent years. It is important to consider the diagnosis by the clinical findings of acute limbic encephalopathy, new onset schizophrenia, or new
onset medically refractory seizures. Identification of the antibodies is important in defining the underlying causes including underlying malignancies.
Antibodies against intracellular proteins are more likely to be paraneoplastic and T-cell mediated. Antibodies against cell surface or synaptic antigens
are mostly B-cell mediated and they are more responsive to immunotherapy. In addition to high index of clinical suspicion, MRI, serum and CSF antibody
testing are most helpful in making the final diagnosis.

Autoimmune encephalitis is a heterogeneous group of
inflammatory central nervous system disorders that attracted more
medical attention over the past decade. The pathophysiological
mechanisms and clinical correlates become better recognized
and understood. The entity was very under-recognized in the past
and believed to be rare. It occurs in people of all ages, the overall
incidence of encephalitis is about 5-8 cases per 100,000 populations
yet no cause could be found in nearly half of these cases [1,2].
Autoimmune encephalitis is believed to be the third leading cause
of encephalitis after viral encephalitis and acute disseminated
encephalomyelitis [1,2]. Early recognition of the diagnosis
followed by specific immunological treatment is important in the
improvement of the outcome of the patients.

As the diagnosis implies, autoimmune encephalitis is an
inflammatory disorder affecting the gray matter or neurons of
the CNS. The clinical presentations will include inflammation
such as low-grade fever, fatigue, headache and malaise. The
predominant clinical features of gray matter involvement will
include various forms of seizures, involuntary movements
such as dystonia, memory impairment, behavioral or psychotic
features and alterations in awareness [2,3]. The most common
site of involvement in the CNS is the limbic system especially the
medial temporal lobes and hippocampus and hence the common
diagnostic term of autoimmune limbic encephalitis. The two most
important clinical categories are autoimmune limbic encephalitis
and immune epilepsy syndromes with medically refractory seizure
as the predominant clinical phenotype.

Autoimmune encephalitis is traditionally classified as
paraneoplastic or non-paraneoplastic depending on whether there
is any identifiable tumor associated antibodies such as anti-Hu
(antineuronal nuclear antibody type 1), anti-Yo, anti Ri and anti-
Ma/Ta etc. [4]. The better and newer classification is to classify the
immune encephalopathy according to the targets of the antibodies,
whether against intracellular constituents or against cell surface/
synaptic antigens. With this newer and better classification, we
understand that the autoimmune encephalitis against intracellular
antigens consist of the classical paraneoplastic diseases as
listed above as well as others such as anti-GAD (glutamic acid
decarboxylase) encephalitis. These diseases are mediated mainly
through a predominant T-cell mediated mechanism with cytotoxic
T cells demonstrated in the pathological specimens because the
neuronal tissues share similar antigenicity with tumour antigens.
These antibodies are generally not pathogenic. On the other hand,
the other class of autoimmune encephalitis are immune mediated
diseases against cell surface and synaptic antigens such as NMDA
(N-Methyl D-Aspartate) Receptor, VGKC (Voltage gated potassium
channel) complex, including LGI1 and CASPR2 antibodies. Other
antibodies also include AMPA receptor, GABA receptor, mGluR5
receptor and DPPX receptor etc. These are real antibody or B-cell
mediated autoimmune encephalitis with real pathogenic antibodies.
Some of these are paraneoplastic e.g. anti-NMDAR encephalitis
complicating ovarian teratoma in young woman, yet many are not
paraneoplastic. These immune encephalitis syndromes often have
relatively specific clinical features that may provide good clues to
the underlying antibody types. Faciobrachial dystonic seizures is a
strong clue to the diagnosis of anti-LGI1 antibody syndrome and
confusion, diarrhoea and weight loss will be very suggestive of
DPPX antibody syndrome [3,4].

Anti-NMDA receptor antibody limbic encephalitis is worth
special mentioning. It is probably the commonest and best
characterized type of autoimmune encephalitis or limbic
encephalitis. It occurs most commonly among young adults and
children. The presentation is often very typical and progression
predictable. It started with a subacute onset in children and young
adults (mean age of around 20) with what appears like a viral
syndrome with low grade fever, headache and malaise. This part
of the illness is often neglected. The patients will then progress to
the next phase of neuropsychiatric presentations of behavioural
disturbances and psychosis. Very often, these patients are admitted
to inpatient psychiatric units as new onset schizophrenia until
they reach the next phase of the illness with speech reduction,
refractory seizures, memory impairment, oral and limb dyskinesias,
autonomic disturbances (bradycardia, fluctuation in temperatures,
salivation and central hypoventilation) and impaired awareness or
coma.

Nearly 60% of young women had an underlying ovarian
teratoma [3]. The prognosis may be surprisingly good when
diagnosis is recognized early and specific treatment including
removal of underlying tumour and immunotherapy instituted
appropriately. It is interesting to note that a proportion of patients
who present with a late “relapse” of herpes simples’ encephalitis
is actually suffering from secondary anti-NMDA limbic encephalitis
with positive anti-NMDA antibody secondary to the herpes simplex
encephalitis [5]. It actually makes real good sense because herpes
virus is a neurotropic virus with special predilection to the limbic
system with glutaminergic neurons and subsequent formation of
anti-NMDA glutamate receptor antibody.

The clue to diagnosis is high index of suspicion. MRI is the most
helpful imaging study showing T2 hyperintensity changes involving
bilateral more than unilateral temporal lobes [6,7]. Normal MRI,
however, does not rule out the diagnosis of autoimmune limbic
encephalitis. CSF will show elevated protein with normal glucose
and mononuclear cell pleocytosis (between 10-100 cells per cubic
mm). Both the serum and CSF may show the presence of specific
antibodies.

Treatment consists of a two-prong approach: immunotherapy
and removal of the immune trigger. If there is an underlying
trigger such as a tumour, removal of the tumour is the best option
in achieving a cure. Immunotherapy consists of corticosteroid,
plasma exchange and intravenous immunoglobulin. If these fail, IV
rituximab may be effective by wiping out the B-cells. Most immune
encephalitis responds well to immunotherapy while spontaneous
remission is very rare [8].

Autoimmune (antibody mediated) encephalitis is an uncommon
yet important cause of acute encephalitis especially limbic
encephalitis. It should be considered in any patient presenting
with new onset acute encephalitis, refractory seizures or psychosis.
Antibodies against intracellular proteins are patho-physiologically
different from antibodies against cell surface or synaptic antigens.
Diagnosis relies on recognition of specific clinical features,
identification of the antibodies in the serum or CSF and aided by
MRI findings. Treatment consists mainly of immunotherapy and
removal of underlying malignancy if present.

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